Conventionally, projection image display devices, by which a small light valve displaying an image in response to a video signal is illuminated and the image is magnified and projected using a projection lens, are known as a method for displaying large-screen videos. Some light valves use a transmissive type or a reflective type liquid crystal panel and some light valves use a digital mirror device, which is an aggregation of micro-mirrors, and projection image display devices using these have been put into practical use. The following is a description of a conventional projection image display device.
FIG. 21 is a conceptual diagram of an optical system showing a projection image display device that uses a conventional columnar optical element (hereafter, “rod integrator”), and a light valve. In this drawing, reference numeral 2 is a lamp, reference numeral 3 is an elliptical concave mirror, reference numeral 4 is a relay lens system, reference numeral 5 is a field lens, reference numeral 6 is a transmissive light valve, reference numeral 7 is a projection lens, and reference numeral 15 is a rod integrator made of a glass material.
The following is a description of the operation. The light-emitting center of the lamp 2 is arranged in the vicinity of a first focal point of the elliptical concave mirror 3. After the light flux emitted from the lamp 2 is reflected by the elliptical concave mirror 3, the light is converged in the vicinity of a second focal point of the elliptical concave mirror 3. The incident face of the rod integrator 15 is arranged in the vicinity of the second focal point. The light flux of the incident light is totally reflected as appropriate by side surfaces in the longitudinal direction of the rod integrator 15 and emitted by the rod integrator 15.
The following is a description of the fundamental operation of the conventional rod integrator 15. FIG. 22 is a top view of the operation of an incident light ray and FIG. 23 is a lateral view of the operation of an incident light ray. In these drawings, the light ray, which is incident at an angle θ, is totally reflected as appropriate by side surfaces in the longitudinal direction of the rod integrator 15. The light ray is transmitted while maintaining its angle, and the light is emitted at an angle θ. Accordingly, if the maximum value of the converging angle of the elliptical concave mirror 3 is 30° for example, a light ray of a maximum 30° corresponding to this is emitted from the rod integrator 15.
Furthermore, if the angles of the incident light rays are different, the number of times the light is totally reflected as appropriate by the side surfaces in the longitudinal direction of the rod integrator 15 is different. Since these are merged at the exit face, the light rays are superimposed at the exit face even when there is an uneven illumination distribution at the incident face. A result of this is that it is possible to obtain an illumination light flux at the exit face of the rod integrator 15 that has superior uniformity and that has a form that is approximately equivalent to a desired illumination range. Note, however, that since better uniformity can be achieved with a larger the number of reflections, a sufficient length of the rod integrator 15 obviously has to be ensured.
Furthermore, the light flux emitted from the rod integrator 15 illuminates the transmissive light valve 6 via a relay lens system 4, which is configured by at least one lens, and a field lens 5. The transmissive light valve 6 displays an image based on an electric signal that is output from a drive circuit (not shown). The image displayed on the transmissive light valve 6 is magnified by a projection lens 7 and projected onto a screen (not shown).
Furthermore, there is a strong demand to make the projected images of such projection image display devices brighter, and projection image display devices have been disclosed that use a plurality of light sources. For example, methods are disclosed such as in Patent Document 1, in which emitted light fluxes from a plurality of light sources are synthesized using a light guiding means such as an optical fiber, and a method in which light sources are arranged in predetermined positions and reflected light is synthesized by a reflective mirror and a reflective prism or the like.
Further still, in Patent Document 2 below, there is one light source, but a tapered portion is formed in the rod integrator that continuously increases in cross section from the incident end face to the exit end face. By controlling the tapering angle of the tapered portion, this structure achieves a desired value in the parallelism of the converged light flux from the lamp.
To increase the brightness in the configuration of the conventional projection image display devices shown above, methods are employed such as raising the lamp power consumption, and using a lamp that is almost a point light source, for example an extra-high pressure mercury lamp with an electrode distance of 1.3 mm or less, and increasing the rate of light convergence to increase the brightness.
However, when using the above two methods, increasing the power consumption while keeping the same electrode distance considerably shortens the life of the light source. Furthermore, leaving the power consumption the same and further shortening the distance between electrodes also results in considerable shortening of the life of the light source. Accordingly, how to further increase device brightness without shortening the life of the light source is an issue in configurations with a single light source such as in Patent Document 2.
On the other hand, a method disclosed in Patent Document 1, which attempts to increase brightness by using a plurality of light sources, is a synthesizing method in which the converging angles of light rays emitted from light source portions, which are made of a light source and an elliptical concave mirror, are left unchanged for emission. For example, when the light fluxes from two light source portions are synthesized, light rays emitted from the elliptical concave mirror with a converging angle of about 15° will have a maximum divergence angle of about 30° that is synthesized and emitted.
For this reason, although it seems to possible to realize a condenser lens to be used at a stage following the synthesizing portion made of a reflective mirror or reflective prism, when trying to achieve a sufficient condensing ratio with the elliptical concave mirror for the converging angle of about 15°, it is necessary that the positions of the first and second focal points of the elliptical concave mirror are sufficiently distanced, and that the elliptical concave mirrors themselves are large, and therefore there is the problem that the device cannot be miniaturized.
Furthermore, presently it is common to use elliptical concave mirrors with a converging angle of approximately 30°, which gives importance to improving brightness and device miniaturization, but when using two of these, the maximum divergence angle corresponding to the converging angle of the light rays reflected from synthesizing portions made of a reflective mirror or reflective prism is about 60°, and it is difficult and impractical to achieve a condenser lens to be used at a stage following the synthesizing portion.
With a configuration of Patent Document 2, the divergence angles at the exit end face can be controlled using a rod integrator with a tapered portion. However, in single light source configurations, this technique is for controlling the parallelism of light fluxes in both horizontal and vertical directions using tapered surfaces formed in the rod integrator in both the horizontal and vertical directions. That is, Patent Document 2 does not disclose a technique addressing the enlargement of the maximum divergence angle when using two light sources.
Patent Document 1                JP H9-50082A        
Patent Document 2                JP H11-142780A        